A nanoscale grapevine with hydrogen grapes could someday provide your car’s
preferred vintage of fuel.
Rice Univ. researchers have determined that a
lattice of calcium-decorated carbyne has the potential to store hydrogen at
levels that easily exceed Department of Energy (DOE) goals for use as a
“green” alternative fuel for vehicles.
The rise of nano-scale strategies for energy storage has been dramatic in
recent years, as evidenced by labs worldwide suggesting various ways to use
nanotubes and ribbons as a medium. But they may not be thinking small enough,
according to new research by the lab of theoretical physicist Boris Yakobson
that was published online in Nano Letters.
Yakobson is Rice’s Karl F. Hasselmann Chair in Engineering and a professor of
materials science and mechanical engineering and of chemistry.
Carbyne is a chain of carbon atoms; it’s what you’d get if you could pull a
string from a slice of graphene the same way you’d pull a loose thread from a
sweater. “A one-atom rod of carbon is as thin as it can ever get, way
thinner than a carbon nanotube,” Yakobson said.
Carbyne is considered an exotic material, but recent experiments show it can
be synthesized and stabilized at room temperature, where the storage is mainly
of interest. That’s important, Yakobson said, because other nanoscale materials
such as carbon nanotubes, graphene and even buckyballs are effective for
hydrogen storage only at conditions that are too cold.
It’s the calcium that serves as a bait and makes room-temperature storage
possible for carbyne. Formed into a lattice, carbyne alone could theoretically
store around 50% of its weight in hydrogen, far above the 6.5% capacity target
set by the DOE for 2015. But the weak binding could work only at very low
temperatures, Yakobson said.
Not so with calcium added. It allows the lattice to adsorb hydrogen with a
binding energy favorable for effective room-temperature, reversible storage.
Because calcium atoms don’t cluster, they can be distributed along the carbyne
strands like grapes on a vine and bind as many as six hydrogen atoms each; this
would give the network a potential storage capacity of about 8 percent of its
weight.
Because a scaffold of single-atom chains would be light and airy, there
would be more room for the hydrogen to aggregate.
Yakobson and his colleagues suggested several scalable strategies for practical
hydrogen storage. In one that resembles the so-called metal organic frames recently
studied by Yakobson’s lab, a diamond-like lattice would allow five hydrogen
atoms to be adsorbed at each calcium atom; the number of carbon atoms in each
strand would determine the total capacity.
In the other, they suggested pulling calcium-decorated strands of atoms from
graphene, which would serve as a frame for the array.
Yakobson said it is difficult to estimate when either of these or some other
realization might happen. “But I am optimistic. From this theoretical
concept, and based on experimental evidence of carbyne synthesis and experience
with metal organic frame architectures, it may take two to three years to
produce carbyne networks and, say, one to two years to tweak the calcium
enrichment to obtain a material with good capacity for hydrogen,” he said.
“So in three to five years, one can have an industrial sample and then
move to scale up—that is, with intense work and some luck.”
